The Metropolitan police was responsible for 120,000 'excessive' stops against people from ethnic minorities in 2008/9, according to the report. Photograph: Andy Rain/EPA

The official equalities watchdog will threaten to brand as racist police forces which are deemed to have used stop and search powers excessively against people from ethnic minorities, the Guardian has learned.

Police forces will be told they face enforcement action unless they give meaningful promises to change, says a report for the Equality and Human Rights Commission expected to be released later this month.

It presents a prima facie case that the police are still failing in their duties under racial equality laws and finds that an officer's power to stop and search, based on having a reasonable suspicion of involvement in criminality, is disproportionately used against Afro-Caribbean and Asian Britons.

For some forces the "disproportionality" is more than 10 times. The report presses the police to defend themselves against the allegation they are breaking the law by highlighting the fact that some forces use the power considerably more than other forces policing the same types of area.

The force identified as the biggest offender and placed under the most pressure by the report is the Metropolitan police, found to be responsible for 120,000 "excessive" stops against those from ethnic minority backgrounds in 2008/9.

A comparison of how frequently the power is used found that the Met uses it up to five times as much as other forces policing urban areas.

The Met carries out 71 stops for every 1,000 people, but the West Midlands force, policing areas with similar issues, carries out just 13 stops per 1,000.

Broken down by race, in one year the Met stopped 195 per 1,000 Afro-Caribbean people, and 78 per 1,000 Asian people. The figure for white people is 49.

The report argues that because so much of the British Afro-Caribbean population live in London, and because the Met uses stop and search so regularly, it skews the national figures.

A draft of the report concludes: "The evidence points to racial discrimination being a significant reason why black and Asian people are more likely to be stop and searched than white people. It implies that stop and search powers are being used in a discriminatory and unlawful way." It finds little merit in arguments advanced to justify excessive use of stop and search against ethnic minority Britons and questions how frequently some forces use the power.

It says the way the power is used has a "small" impact in tackling crime while inflicting damage on community relations.

Within the commission there has been debate about how strong the conclusion should be and how tough the action should be against the police.

The report covers only stop and searches carried out when an officer has a reasonable suspicion of an individual's involvement in criminality, which are covered by the Police and Criminal Evidence Act.

It does not cover stops where no reasonable suspicion is needed, such as under section 60 of the Public Order Act, where some studies have found ethnic minority people are targeted even more, nor does it cover stops under counter-terrorism powers, though notes concern about those.

Last week the independent reviewer of terrorism legislation, Lord Carlile, said counter-terrorism stops should be ended because they barely make a dent against extremists but fuel resentment against the police.

Away from London, the Dorset and Hampshire forces are over 10 times more likely to stop black people than white people. South Yorkshire and Thames Valley are the most likely to stop British Asians compared with white people.

The report does praise some sections of the police and identifies that some forces have seemingly wiped out racial discrimination in the way they use stop and search powers.

It also cites the example of one force, Cleveland, which in a decade slashed its use of stop and search to one-fifth of its previous level, and achieved falling crime rates and one of highest levels of public confidence in the country.

The Stoke division of Staffordshire police managed to cut its rate of "disproportionality" to one-third of its previous level after enacting reforms and saw its crime rate fall.

In a parliamentary briefing in January 2010 the commission said there were approximately a million stops and searches every year.

It said: "This specifically must not be based on generalisations, for example, on grounds of race or appearance, or people's past record, but only on suspicious behaviour or matching a specific witness description."